1. Field of the Invention
This invention relates to implantable blood filters and more particularly to vena cava filters which resist migration once they have been emplaced in a vessel.
2. Prior Art
Blood clots (emboli) carried in the blood stream often constitute serious threats to health and in some instances, to life itself. The reduction of such clots, or their stabilization and arrest of further migration in the circulatory system of the body, are desiderata constantly motivating the development by the medical profession of new techniques and devices for this purpose. Although emboli moving in other portions of the circulatory system can also present serious problems, development of means for preventing emboli from migrating into the pulmonary circulation from the vena cava has received the primary attention.
Ligation of the vena cava was an early technique to minimize movement of emboli therein, with collateral circulation being relied upon to provide adequate venous return of blood to the heart. This procedure, which involved major abdominal surgery, progressed to the utilization of harpstring filters; staple plication; smooth, serrated and channeling external clips evolving into intravascular springs; balloons; and filters. The utilization of filters emplaced in the vascular system provides the obvious advantage over ligation of major blood vessels such as the vena cava, of not requiring general anaesthesia surgery and laparotomy.
U.S. Pat. No. 3,540,431 discloses an intravascular filter for entrapment and arresting of emboli, advanced by Mobin-Uddin and associates. The Mobin-Uddin filter is an umbrella type structure which includes a plurality of expanding struts or ribs which carry points at the divergent ends thereof which impale or engage the vessel wall when the filter is in its expanded state. This device has had problems with migration, and it was withdrawn from the market in 1986.
U.S. Pat. No. 3,952,747 to Kimmell, Jr., incorporated, herein by reference, discloses a vena cava filter as well as a method and apparatus for percutaneous insertion of that filter into a human. The Kimmel disclosure defines the filter as having a plurality of convergent legs disposed in a generally conical array and joined at their convergent ends to an apical hub, each leg having a reversely bent hook at is distal end.
An advancement of the Kimmel, Jr. reference is shown in U.S. Pat. NO. 4,817,600 to Herms et al, also incorporated herein by reference, which discloses a titanium filter having a plurality of legs joined to a head or nose bead, the legs having a first straight portion, and sharply divergent legs extending therefrom.